Centrifugal compressors are useful in a variety of devices that require a fluid to be compressed. The devices include, for example, turbines, pumps, and chillers. The compressors operate by passing the fluid over a rotating impeller. The impeller works on the fluid to increase the pressure of the fluid. Because the operation of the impeller creates an adverse pressure gradient in the flow, many compressor designs include a diffuser positioned at the impeller exit to stabilize the fluid flow.
It is often desirable to vary the amount of fluid flowing through the compressor or the pressure differential created by the compressor. However, when the flow of fluid through the compressor is decreased, and the same pressure differential is maintained across the impeller, the fluid flow through the compressor often becomes unsteady. Some of the fluid stalls within the compressor and pockets of stalled fluid start to rotate with the impeller. These stalled pockets of fluid are problematic in that they create noise, cause vibration, and reduce the efficiency of the compressor. This condition is known as rotating stall or incipient surge. If the fluid flow is further decreased, the fluid flow will become even more unstable, in many cases causing a complete reversal of fluid flow. This phenomenon, known as surge, is characterized by fluid alternately surging backward and forward through the compressor. In addition to creating noise, causing vibration, and lowering compressor efficiency, fluid surge also creates pressure spikes and can damage the compressor.
A solution to the problems created by stall and surge is to vary the geometry of the diffuser at the exit of the impeller. When operating at a low fluid flow rate, the geometry of the diffuser can be narrowed to decrease the area at the impeller exit. The decreased area will prevent the fluid stalling and ultimately surging back through the impeller. When the fluid flow rate is increased, the geometry of the diffuser can be widened to provide a larger area for the additional flow. The variable geometry diffuser can also be adjusted when the pressure differential created by the compressor is changed. When the pressure differential is increased, the geometry of the diffuser can be narrowed to decrease the area at the impeller exit to prevent fluid stall and surge. Similarly, when the pressure differential is decreased, the geometry of the diffuser can be widened to provide a larger area at the impeller exit.
Several devices for varying the geometry of the diffuser are disclosed in the prior art. For example, U.S. Pat. No. 5,116,197 to Snell discloses a variable geometry diffuser for a variable capacity compressor. This device, and others like it, include a moveable drive ring that may be selectively adjusted to vary the geometry of the diffuser at the impeller exit. The ring is positioned adjacent to one wall of the diffuser and can be moved out into the flow of fluid to decrease the area of the diffuser to account for a lower fluid flow or an increased pressure differential.
When the ring is positioned in the fluid flow, the known devices create an opening between the ring and the wall into which fluid exiting the impeller will flow. When attempting to move the ring out of the fluid flow, the fluid must be cleared from between the ring and wall. Displacing this fluid so the ring can be moved requires a significant amount of force, since the fluid acts to oppose the motion of the wall.
Devices such as set forth in Snell are expensive, as the drive ring pilots on a nozzle base plate. The nozzle base plate includes precision-machined tracks machined into its cylindrical outer surface. The drive ring includes corresponding spherical pockets on its inside diameter. Balls are mounted between the nozzle base plate and the drive ring, sliding in the tracks and pockets, the arrangement converting the rotational movement of the drive ring into axial movement while preventing the drive ring and the nozzle base plate from becoming disconnected. This assembly, however, is expensive to fabricate, as close tolerances must be maintained between the inner diameter of the drive ring and the outer diameter of the nozzle base plate. In addition, the spherical pockets on the drive ring must be matched to the tracks on the nozzle base plate. Furthermore, wear will ultimately result in the replacement of both the drive ring and the nozzle base plate.
Another approach is set forth in Publication US 2002/0014088A1 to Seki et al. In this approach, the ring which is positioned in the fluid flow is supported by the casing. Three protrusions from the casing are fitted into grooves on the outer peripheral face of the diffuser ring. A bearing may be used with each protrusion to suppress rubbing contact between the casing and the diffuser ring. The diffuser ring is connected to a shaft. Rotation of the shaft causes the diffuser ring via a bracket to rotate in the circumferential direction. The circumferential movement causes the diffuser ring to move axially as the protrusions guide the axial movement of the diffuser ring along the grooves. While effective, the approach is expensive, as the protrusions must be accurately placed in the casing. The threaded shaft and motor for shaft rotation also add expense to this assembly.
In light of the foregoing, there is a need for a variable geometry diffuser for a variable capacity compressor that may be easily opened and closed during the operation of the compressor. The variable geometry diffuser should be inexpensive to manufacture, easy to assemble, simple to repair or replace and provide positive engagement for accurate position determination in response to signals or commands from the controller.